Implantable cardiac stimulators are in common use in the form of cardiac pacemakers or cardioverters/defibrillators. Such cardiac stimulators are used in treating disorders in the natural contraction of the chambers of a heart with the assistance of stimulation pulses and/or defibrillation shocks. Thus, such cardiac stimulators are usually connected to electrode lines having stimulation or defibrillation electrodes in a chamber of a heart, or in the immediate vicinity thereof.
In the chamber of a heart, a localized excitation of the cardiac muscle tissue (myocardium) propagates from an excitation site into the myocardium via conduction, and results in depolarization of the muscle cells, and thus in a contraction of the myocardium. After a brief period of time repolarization of the muscle cells, and thus relaxation of the myocardium, occurs. During the depolarization phase the cardiac muscle cells are insensitive to excitation, i.e., are refractory. The electrical potentials accompanying the depolarization and repolarization may be detected, and their variation over time, referred to as an electrocardiogram, may be evaluated.
Such natural (intrinsic) cardiac behavior events are detected by measuring the electrical potentials of the myocardium in one or more cardiac chambers, using sensing electrodes provided on one or more corresponding electrode lines. The sensing electrodes may also serve as stimulation electrodes, and may switch between use as stimulation electrodes and sensing electrodes. Typically, for the sensing, i.e., detection, of intrinsic events, a sensing electrode pair is provided which is formed by two adjacent electrodes, usually a tip electrode and an annular electrode, wherein the tip electrode is also used as a stimulation electrode. In this manner a bipolar measurement of an intracardiac electrocardiogram (IEGM) is obtained.
The electrodes typically detect electrical signals in the cardiac chamber in which the electrodes are situated. Detection of so-called “far-field electrocardiograms” is also possible, i.e., detection of electrical potentials of the myocardium whose origin is remote from the position of the electrode. For example, an electrode placed in the ventricle may also be used to detect signals from an atrium. Far-field electrocardiograms may be recorded using not only the electrodes provided for stimulation, but also by use of the shock coils provided for the delivery of defibrillation shocks. The opposite pole for signal detection may be another electrode, another shock coil, and/or the electrically conductive housing of an implantable cardiac stimulator.
During operation of the cardiac stimulator, the sensing electrodes are connected to corresponding sensing units which are designed to evaluate an electrocardiogram recorded via the sensing electrode(s), and in particular to detect intrinsic atrial or ventricular events, i.e., natural atrial or ventricular contractions. This is carried out, for example, by threshold value comparison, wherein an intrinsic event is detected within an intracardiac electrocardiogram when the voltage therein exceeds a suitably specified threshold value. The intrinsic atrial heart rate (atrial frequency) or ventricular heart rate (ventricular frequency) may be determined from the time sequence of atrial or ventricular events within the electrocardiogram.
Apart from the previously described characteristics of a cardiac pacemaker, an implantable cardioverter/defibrillator can also deliver a stronger current pulse to the heart, one which doesn't merely stimulate (depolarize) a small portion of the myocardium, and which instead depolarizes the greatest possible portion of the myocardium (and thus makes it refractory) in order to interrupt circulating excitation of the myocardium that is typical of fibrillation. Such a pulse is referred to as “defibrillation shock.” This type of pulse is typically delivered via a defibrillation electrode having a large surface area compared to the stimulation or sensing electrode.
Such a defibrillation electrode is frequently implemented in the form of a shock coil on the exterior of the electrode line in the particular cardiac chamber. For example, besides a tip electrode or annular electrode for the stimulation and sensing, a ventricular electrode line may also have a ventricular shock coil, as well as a proximal shock coil located in the superior vena cava after implantation.
A defibrillation shock is generally delivered when the cardiac stimulator detects fibrillation (also referred to as ventricular fibrillation), i.e., irregular high-frequency intrinsic activity of the heart, which results in incomplete contraction of the affected cardiac chamber. Such fibrillation is included in tachycardia arrhythmia, which in addition to fibrillation also includes tachycardia (for example, ventricular flutter). In contrast to fibrillation, tachycardia routinely results in complete contraction of the affected cardiac chamber, but at a rate that is higher than physiologically appropriate. Such tachycardia may often be treated by antitachycardia stimulation, without the need for a defibrillation shock. Fibrillation is generally treated by defibrillation shock.
For detection of ventricular fibrillation, a detector is typically provided as a component of a control unit for the cardiac stimulator, and is connected to the right ventricular sensing unit and is designed to detect primary ventricular fibrillation on the basis of a right ventricular, bipolarly derived intracardiac electrocardiogram (IEGM). If a specified detection condition is met (e.g., when X of Y criteria are met), the detector indicates ventricular fibrillation.
For tachycardia of the ventricle, a distinction is made between conducted supraventricular tachycardia (SVT), which originates in the atrium, and ventricular tachycardia (VT), which originates in the ventricle itself. The type of ventricular tachycardia (ventricular tachycardia (VT) or supraventricular tachycardia (SVT)) is important for the treatment initiated after tachycardia is detected.
Tachycardia and fibrillation may be treated in various known ways by use of intracardiac electrotherapy. Antitachycardia stimulation (antitachycardia pacing, ATP) is often provided in the form of overdrive stimulation, in which stimulation pulses are delivered at a stimulation rate that is increased compared to the existing intrinsic (tachycardial) heart rate. Overdrive stimulation is intended to interrupt a reentry cycle of excitation of the myocardium, which is typical for ventricular tachycardia (VT, ventricular flutter), by means of a stimulation pulse provided before the natural (intrinsic) excitation of the affected cardiac chamber. For this purpose it is necessary to reliably detect intrinsic ventricle contractions before the pulse is initiated. Antitachycardia stimulation can also be provided via the delivery of a cardioversion shock or defibrillation shock, wherein the former usually has lower energy than the latter. Defibrillation shock is intended to make the entire myocardium of an affected cardiac chamber refractory at the same time, and thus to make the myocardium temporarily insensitive to excitation in order to thus interrupt circulating excitation of the affected cardiac muscle.
Previous methods for discriminating between ventricular tachycardia (VT) and supraventricular tachycardia (SVT) in single-chamber ICDs often result in inadequate therapies, because the intracardiac electrocardiograms (IEGMs) are evaluated only with respect to the intervals or changes thereto. Maximum sensitivity is always a priority—it is preferable to always detect actual VTs—and the specificity (detection of SVTs) is often unsatisfactory in prior methods. SVTs which are incorrectly classified as VTs result in a considerable number of inadequate defibrillator shocks, which are a physical and emotional burden on the affected patients. In addition, when ATP treatments are inadequate there is a risk of triggering dangerous ventricular tachycardia.
By use of morphological methods, the specificity of the SVT/VT discrimination may be improved without significantly limiting the sensitivity.